Extreme ultraviolet (EUV) lithography (EUVL) is a promising future lithography technique. EUV light may be produced using a small, hot plasma that will efficiently radiate at a desired wavelength, e.g., in a range of approximately 11 nm to 15 nm.
Reflective masks may be used in EUVL. EUV light is strongly absorbed by many materials, so material choice is an important consideration in EUVL mask design. EUVL masks may include a reflective mask blank covered by a patterned layer of an EUV absorber material. The EUVL mask blank may include multilayer (ML) reflective coatings, e.g., alternating layers of molybdenum (Mo) and silicon (Si), on a substrate.
A capping layer may be provided on the top of the mask blank to protect the ML. A layer of silicon may be used as the capping layer. Since silicon has relatively low EUV absorption, a relatively thick layer of silicon (e.g., 10-20 nm) may be used without significantly sacrificing the reflectivity of the mask blank.
A problem with silicon as a capping material is that it tends to oxidize rather easily. The native oxide may be removed during multiple cleanings. Re-growing of native oxide and consequently removing it during cleaning over several cycles may substantially thin the silicon capping layer, reducing the capping layer's ability to protect the underlying ML from damage. The reflectivity of the blanks may also vary over a relatively large scale as the thickness of the silicon capping layer becomes thinner and non-uniform.